Krassner was a child violin prodigy and was the youngest person ever to play Carnegie Hall, in 1939 at age six.[2] His parents were Jewish,[3][4] but Krassner is firmly secular, considering religion "organised superstition."[5] He majored in journalism at Baruch College (then a branch of the City College of New York) and began performing as a comedian under the name Paul Maul. He recalled:

While in college, I started working for an anti-censorship paper, The Independent. After I left college I started working there full time. So, I never had a normal job where I had to be interviewed and wear a suit and tie. I became their managing editor and also did freelance stuff for Mad magazine. But Mad was aimed at a teenage audience, and there was no satirical magazine for adults. So it was a kind of organic evolution toward The Realist, which was essentially a combination of satire and alternative journalism.[6]

In 1963, he created what Kurt Vonnegut described as "a miracle of compressed intelligence nearly as admirable for potent simplicity, in my opinion, as Einstein's e=mc2." Vonnegut explained: "With the Vietnam War going on, and with its critics discounted and scorned by the government and the mass media, Krassner put on sale a red, white and blue poster that said FUCK COMMUNISM. At the beginning of the 1960s, FUCK was believed to be so full of bad magic as to be unprintable. [...] By having FUCK and COMMUNISM fight it out in a single sentence, Krassner wasn't merely being funny as heck. He was demonstrating how preposterous it was for so many people to be responding to both words with such cockamamie Pavlovian fear and alarm."[8][9]

In 1971, five years after Lenny Bruce's death, Groucho Marx said, "I predict that in time Paul Krassner will wind up as the only live Lenny Bruce."[7]

The Realist was published on a fairly regular schedule during the 1960s, then on an irregular schedule after the early 1970s. In 1966, Krassner published The Realist's controversial "Disneyland Memorial Orgy" poster, illustrated by Wally Wood, and he recently made this famed black-and-white poster available in a digital color version. The Realist also distributed a red, white and blue Cold Warbumper sticker that read "Fuck Communism."

Krassner's most notorious satire was the article "The Parts That Were Left Out of the Kennedy Book", which followed the censorship of William Manchester's book on the Kennedy assassination, The Death of a President. At the climax of the grotesque-genre short-story, Lyndon B. Johnson is described as having sexually penetrated the bullet-hole wound in the throat of John F. Kennedy's corpse.[10] According to Elliot Feldman, "Some members of the mainstream press and other Washington political wonks, including Daniel Ellsberg of Pentagon Papers fame, actually believed this incident to be true."[11] In a 1995 interview for the magazine Adbusters, Krassner commented: "People across the country believed – if only for a moment – that an act of presidential necrophilia had taken place. It worked because Jackie Kennedy had created so much curiosity by censoring the book she authorized – William Manchester's 'The Death Of A President' – because what I wrote was a metaphorical truth about LBJ's personality presented in a literary context, and because the imagery was so shocking, it broke through the notion that the war in Vietnam was being conducted by sane men."[12]

In 1966, he reprinted in The Realist an excerpt from the academic journal the Journal of the American Medical Association, but presenting it as original material. The article dealt with drinking glasses, tennis balls and other foreign bodies found in patients' rectums.[13] Some accused him of having a perverted mind, and a subscriber wrote "I found the article thoroughly repellent. I trust you know what you can do with your magazine."[13]

Krassner revived The Realist as a much smaller newsletter during the mid-1980s when material from the magazine was collected in The Best of the Realist: The 60's Most Outrageously Irreverent Magazine (Running Press, 1985). The final issue of The Realist was #146 (Spring, 2001).

In 1962 Paul published an anonymous interview with Dr Robert Spencer detailing his involvement in illegal but safe abortions.[15] Subsequent to the publication Paul received calls from women asking to be put in contact with the interviewee. Paul was later subpoenaed to appear before grand juries investigating abortion crime.

In 1965 Paul contributed to the Free University of New York a lecture entitled "Why the New York Times is funnier than Mad Magazine".[16] In 1968, Krassner signed the “Writers and Editors War Tax Protest” pledge, vowing to refuse tax payments in protest against the Vietnam War.[17]

Krassner is the only person to win awards from both Playboy magazine (for satire) and the Feminist Party Media Workshop (for journalism). He was the first living man to be inducted into the Counterculture Hall of Fame,[25] which took place at the Cannabis Cup in Amsterdam. He received an American Civil Liberties Union Uppie (Upton Sinclair) Award for dedication to freedom of expression, and, according to the FBI files, he was described by the FBI as "a raving, unconfined nut."[7][26]George Carlin commented: "The FBI was right, this man is dangerous – and funny; and necessary."[7] In 2005 he received a Grammy nomination for Best Album Notes for his essay on the 6-CD package Lenny Bruce: Let the Buyer Beware.

Krassner was harshly criticized, along with many males on the Left, in Robin Morgan's classic feminist manifesto, "Goodbye to All That":[27][28][29][30]

"Goodbye to lovely "pro-Women's Liberationist" Paul Krassner, with all his astonished anger that women have lost their sense of humor "on this issue" and don't laugh any more at little funnies that degrade and hurt them: farewell to the memory of his "Instant Pussy" aerosol-can poster, to his column for the woman-hating men's magazine Cavalier, to his dream of a Rape-In against legislators' wives, to his Scapegoats and Realist Nuns and cute anecdotes about the little daughter he sees as often as any properly divorced Scarsdale middle-aged father; goodbye forever to the notion that a man is my brother who, like Paul, buys a prostitute for the night as a birthday gift for a male friend, or who, like Paul, reels off the names in alphabetical order of people in the women's movement he has fucked, reels off names in the best locker-room tradition—as proof that he's no sexist oppressor."